In the beginning of Cinder, Sunto's mother becomes infected with letumosis. Sunto tries to go near his mother, but she tells him to back away, afraid that he would get the plague, too.

Much later in the book, when visiting the quarantine for those infected with letumosis, Cinder plans on giving the antidote to her stepsister, Peony, but Peony passes away before being able to drink the antidote. As Cinder is leaving, she sees Sunto, who has also gotten infected by the plague, and she decides to give the antidote to him. Near the end of the book, Sunto becomes famous for miraculously healing from letumosis.